1. Field of the Invention
The present invention relates to a system for eliminating air from liquid-carrying conduits, in particular in a photoresist dispensing system. It relates more particularly to an air eliminating system in which there is a reduced possibility of contamination by residual resist in the conduits after the air elimination process has been carried out, as well as other improvements.
2. Background Art
Photoresist dispensing systems are employed to supply photoresist to equipment which utilizes a photographic process for manufacturing integrated circuits. A common problem in such systems is how to handle excess air, nitrogen, or other gas which either outgasses from the photoresist or is introduced into the system in another way, such as when changing bottles. Microscopic gas bubbles can block key filter elements or cause foaming of the resist as it is pumped through the system, impairing the quality of the dispensed resist. Larger quantities of gas can totally interrupt the flow of the resist. In either case, lengthy manual procedures are needed to clear excess gas from the system, and these procedures are not always successful.
A known photoresist dispensing system is the PDS-1000 manufactured by the Iwaki Walchem Corporation in Holliston, Mass., which includes a PDS-1300 automatic air elimination module, shown in FIG. 1. The Iwaki system attempts to solve some of these problems by automatically changing bottles and performing some gas elimination steps, but leaves other problems unsolved.
As shown in FIG. 1, if the Iwaki system comprises a pair of resist bottles B1 and B2 connected to a trap tank T via a normally open valve BV1 and a normally closed valve BV2, respectively. The trap tank holds approximately a 10-minute supply of resist and supplies the resist via a pump P to a spin unit SU which is part of an integrated circuit manufacturing system.
When the first bottle B1 becomes empty, the resist level in the trap tank drops and air enters the trap tank. A sensor S2 turns off, in response to the absence of resist, indicating that there is now air at the top of the trap tank, and issues a signal which causes the valve BV1 to close and the valve BV2 to open, and withdrawal of resist begins from the second bottle B2. Air can enter the system both from the empty bottle B1 and its feed lines, and from the empty feed lines that run to the bottle B2.
A sensor S3 is provided for detecting when the resist level has reached the bottom of the trap tank in order to initiate an emergency shutdown procedure. To attempt to eliminate the excess air which has entered the system from the bottles and their associated conduits, as well as any gasses which have outgassed from the resist, an air elimination cycle then begins. A normally closed air-operated valve AV4 is opened, to permit excess air and photoresist to be drawn from the trap tank. The valve AV4 is located directly at the top of the trap tank. A vacuum generator V, which will generate vacuum to drive the air elimination cycle, receives pressurized air from a utility air line controlled by a solenoid valve SV5. (Hereinafter the abbreviations AV and SV indicate an air-actuated valve and a solenoid-actuated valve, respectively.) The vacuum generator includes a conventional combination of a check valve and a needle valve to form a speed controller SC. Vacuum is generated by an injector having a Venturi with a 1.0-mm orifice, which creates a vacuum in a vacuum chamber to draw the excess air and resist through a vacuum line VL from the trap tank via the valve AV4. The pressurized air is exhausted from the vacuum generator via a normally closed valve AV1 which has been opened for this purpose. The presence of positive and negative air pressure at the various points in the vacuum generator V is indicated by circled plus-signs and minus-signs in the figures.
In the air elimination cycle, which attempts to eliminate excess gas from the trap tank and from the lines between the new bottle B2 and the trap tank, excess gas and resist are drawn by the vacuum into the line VL. When a sensor S1 at the vacuum chamber detects the new photoresist which has been drawn from the second bottle B2, the air eliminator cycle is ended by closing valve AV1 to turn off the vacuum generator, and by closing valve AV4. The line VL, and the conduit C are then full of resist, whereby excess air and gas have been eliminated.
During the foregoing air elimination sequence, a normally open drain valve AV3 is kept closed. The drain valve AV3 is also closed during normal operation. It is opened only during the drainage cycle to be described below.
After the air elimination sequence, a drainage sequence is performed to rid the system of excess resist. The valve AV3 is opened and pressurized air is admitted from the utility air line via valve SV5. No vacuum is generated because valve AV1 is closed. Instead, the air pressurizes the vacuum chamber and pushes the resist back through the line VL, the drain line DL and the valve AV3 and into the drain.
The Iwaki system also has a valve AV2 which is connected at one end to the input of the drain valve AV3 but has no outlet connection and no designated function.
Table 1 illustrates the foregoing steps in more detail. Step 1 represents the normal operation of the resist supply, which is accordingly referred to as the "standby mode" of the air elimination module. During step 1, the Iwaki system supplies air to valve AV1 to maintain it open. This feature of the Iwaki system is undesirable. Valve AV1 should not be kept open because that creates the possibility of vacuum being inadvertently generated at an inappropriate time, or of pressurized gas being admitted into the module from an exhaust manifold downstream from the valve AV1.
Further, in step 1, air pressure is supplied continuously to valve AV3 to keep it closed, which is necessary because valve AV3 is a normally open valve in the Iwaki system.
At step 2, sensor S2 has turned off, indicating the detection of air at the top of the trap tank T. The air injector valve SV5 is turned on to admit air to the vacuum generator; the trap tank valve AV4 is also turned on; and the air elimination cycle is begun in order to eliminate the air that has been detected. The signal from sensor S2 also initiates the switchover from bottle B1 to bottle B2. However, the bottles are not switched if the S2 signal does not continue for a time T2 (here, 30 seconds), to prevent the switchover of the bottles from being initiated by the detection of a small quantity of air, when the first bottle is not in fact empty.
The remaining steps in Table 1 have been described previously or are self-explanatory in view of the comments in the table.
The Iwaki PDS-1000 system is incorporated by reference as known prior art. It has several important defects. After the drainage cycle, in which pressurized air from the utility air line has been utilized to force the resist out of the vacuum chamber and back to the drain through the vacuum line VL, the drain line DL, and the valve AV3, resist may still remain in the conduit section C, which extends approximately 8 to 10 inches (20 to 25 cm) from the valve AV4 to the junction of the drain line DL and the line VL, and/or in other lines. This remaining resist can bubble during the next vacuum cycle and create a mist which will clog the valve AV1 so as to prevent it from opening fully, reducing the vacuum generated and thereby interfering with or slowing down the vacuum cycle. If the air elimination sequence is not completed within a fixed time, because of clogging or another malfunction in the Iwaki system, the system automatically shuts down and must be manually restarted.
Further, if valve AV1 does become clogged and the pressure on its input side is high enough, air can be forced into the top of the vacuum chamber, causing a backflow of air, back through the trap tank T and into the source bottle B2. Such an occurrence can completely drain the Iwaki system of all resist.
In a busy Iwaki installation it is common for the valve AV1 to clog almost daily, requiring the system to be shut down so that the valve AV1 can be cleaned with a solvent or detergent. Clogging of the valve AV1 is a significant problem which increases downtime, reduces production, and substantially impairs the usefulness of the entire photoresist dispensing system.
A further defect in the Iwaki system is a result of the valve AV3 being normally open. It is only closed when the air elimination module of FIG. 1 is in its standby mode, i.e., during normal resist-supplying operation of the system. In a normal installation, a plurality of air elimination modules are usually interfaced to a common drain manifold. If power to the drain valve AV3 in a first module happens to be lost, that valve AV3 will open. If a drainage cycle then happens to be under way in another module, resist and/or air from the drain valve AV3 of the second module will back up into the inadvertently open valve AV3 in the first module. This sequence of events requires the first module to be shut down for either manual cleaning or activation of the air elimination and drainage sequences.
Another disadvantage of the Iwaki system is the use of a large 1.0 mm orifice in the vacuum generator. The large 1.0 mm orifice in the Venturi provided by Iwaki can cause an excessive pressure drop on a typical main air line, even though the air line complies with the specifications of the Iwaki system. Such pressure drop can cause all the valves to malfunction.